A method that effectively corrects PA nonlinearity, adaptive digital predistortion (DPD) technology has now been widely applied in wireless communication transmission systems.
FIG. 1 is a block diagram illustrating the principles of a known PA system. As shown in FIG. 1, after passing through a digital predistorter 102, a predistorted signal is obtained for baseband signal source information x(n) from signal source 101, and the predistorted signal is converted into an analogue signal via a digital-to-analogue (D/A) converter 103. After this analogue signal is up-converted into a radio frequency signal via up-frequency converter 104, it is inputted into an analogue variable gain attenuator (VGA) 105, whereby it is gain-adjusted and then input into a power amplifier 106. The signal amplified by power amplifier 106 is transmitted via an antenna. At the same time, a part of the signal output from the power amplifier 106 is down-converted via a down-frequency converter 107 and then fed back to an analogue-to-digital converter 108, whereby it is sampled by the analogue-to-digital converter 108 to obtain a feedback PA output digital baseband signal. This baseband signal is analyzed using digital signal processing technology in an out-band power calculating module 109 to obtain an out-band power value of this digital baseband signal. Thereafter in a predistorter coefficient updating unit 110, the out-band power value is optimized as a target function, and the coefficient of the predistorter is updated using an updating algorithm, to thereby achieve the process of adaptive predistortion.
There is usually demand for power control in any communication system. Power control is generally achieved by adjusting the analogue variable gain attenuator (VGA) which is placed upstream of PA. This means that the output from predistorter 102 should be first regulated via the variable gain attenuator in order to be input to PA. Although power control does not change the real characteristics of PA, analogue link nonlinearity downstream of the output terminal of the predistorter is changed relative to the output terminal of the predistorter. Thus, once power control is carried out, it is required for the predistorter 102 to be regulated in real time to track the change in nonlinearity. But it takes some time for convergence of the updating algorithm in the adaptive predistorter coefficient updating unit 110, and the time for convergence is particularly longer when a scalar predistorter is used—when the communication system performs continuous and fast power control, it is difficult for the predistorter 102 and the predistorter coefficient updating unit 110 to catch up with the change in link nonlinearity, whereby performance of the system is deteriorated.